Purely electromechanical relays are often used so that a relatively low power control signal can be used to control a higher power signal. An electromagnetic coil is energized by the control signal to cause an armature to change position. The armature may carry one or more movable contacts. A frame carries one or more corresponding fixed contacts. Accordingly, pairs of contacts serve as switches to control the higher power signal based upon movement of the armature relative to the frame.
Solid-state relays are also available to perform a similar isolation function. A solid-state relay may include a driver circuit and a field-effect transistor (FET) providing the output signal and having a control terminal or gate connected to the driver circuit. The driver circuit receives the control signal and operates or drives the output FET based upon the control signal.
One disadvantage of a solid-state relay is that it typically does not have non-volatile memory to retain its on/off state when power is discontinued. In addition, a solid-state relay also typically requires a continuous on/off control signal, and is, therefore, not compatible with a typical spacecraft interface, for example, that uses 28-volt pulses for on/off signals.
A mechanical latching relay could provide state-retention even when power is removed. Unfortunately, a typical medium/high current space-qualified mechanical latching relay is physically large, and not amenable to use in high-density circuit board applications.
In addition, typical space applications avoid a mechanical latching relay due to concern for wearing out of the contacts when large currents are being switched. To overcome this drawback, a circuit can be implemented where the switching is not done under load, that is, where cold switching is designed into the circuit. This can complicate circuit design and potentially affect reliability.
Some hybrid electromechanical/solid-state relays have been developed including a solid-state power switching device connected across the output contacts of the relay to reduce the switching load on the contacts, as disclosed, for example, in U.S. Pat. No. 6,078,491 to Kern et al. and U.S. Pat No. 5,699,218 to Kadah. U.S. Pat. No. 5,053,907 to Nishi et al. discloses sequential operation of three sets of contacts along with power devices to reduce arcing and resultant damage that may otherwise occur. These approaches may still be unsuitable for certain applications, particularly, spaceborne applications and where non-volatile state retention is required.